This invention relates generally to methods for the recovery of uranium and, more particularly, to the recovery of uranium from particulate uranium tetrafluoride. The invention was made as a result of a contract with the United States Department of Energy.
In a process for the production of uranium fuel cores for use in nuclear reactors, uranium ore concentrates are converted to highly pure UF.sub.4, which is reduced to uranium metal suitable for the fabrication of cores. More specifically, the ore concentrates are treated with nitric acid, forming a solution of uranyl nitrate. The solution then is contacted with an organic solvent (tributyl phosphate and kerosene) to extract the uranium, which is then reextracted into water. The resulting uranyl nitrate solution is heated to effect conversion of the uranium to UO.sub.3. The UO.sub.3 then is reduced to UO.sub.2, which is hydrofluorinated to UF.sub.4. The UF.sub.4 then is reduced to metal uranium suitable for fabrication into the cores.
In the course of the above-described process, the UF.sub.4 sometimes is contaminated with UO.sub.2 F.sub.2, other uranium oxides, or metallics (e.g., Ni and Fe) to the extent that it is not within process specifications. It is highly desirable that the uranium in the off-specification material be reclaimed for re-introduction into the process. Over the past thirty years, two reclamation processes has been employed occasionally, but neither has been very satisfactory. One of these processes entails pyrohydrolysis of the contaminated UF.sub.4. That is, the off-specification UF.sub.4 is exposed to steam and air at temperatures in the range of from 1500.degree. F. to 1900.degree. F., converting the UF.sub.4 to HF and highly pure U.sub.3 O.sub.8. The U.sub.3 O.sub.8 is recycled to the above-mentioned ore-dissolution step of the above-described process. Unfortunately, the pyrohydrolysis operation is subject to several disadvantages: it involves the generation of corrosive HF, it requires very high temperatures, and it is highly energy inefficient.
The other reclamation process entails dissolution of the contaminated UF.sub.4 in hot concentrated nitric acid, to which alumina has been added to complex the corrosive fluoride as AlF.sub.3. The resulting uranyl nitrate and AlF.sub.3 are introduced to the above-mentioned extraction system. The resulting purified uranyl nitrate is converted to highly pure UO.sub.3 and recycled to the UF.sub.4 -production process. This mode of reclamation also is subject to several disadvantages: it is expensive, objectionable NO.sub.x is generated as a byproduct, and alumina may include crystallization in the digest liquor, limiting the amount of UF.sub.4 being digested.